Recent studies have shown that ternary and quaternary compounds having the general formula Mn+1AXn exhibit unusual and exceptional mechanical properties as well as advantageous electrical, thermal and chemical properties. Despite having high stiffness these ceramics are readily machinable, resistant to thermal shock, unusually damage tolerant, have low density and are thermodynamically stable at high temperatures (up to 2300° C.).
Mn+1AXn compounds have layered and hexagonal structures with M2X layers interleaved with layers of pure A and it is this structure, comprising exceptionally strong “metallic” M-X bonds together with relatively weak M-A bonds, which gives rise to their unusual combination of properties.
Mn+1AXn compounds are characterized according to the number of transition metal layers separating the A-group element layers: in “211” compounds there are two such transition metal layers, in “312” compounds there are three and in “413” compounds there are four. 211 compounds are the most predominant, these include Ti2AlC, Ti2AlN, Hf2PbC, Nb2AlC, (Nb,Ti)2AlC, Ti2AlN0.5C0.5, Ti2GeC, Zr2SnC, Ta2GaC, Hf2SnC, Ti2SnC, Nb2SnC, Zr2PbC and Ti2PbC. The only known 312 compounds are Ti3AlC2, Ti3GeC2, Ti3SiC2. Ti4AlN3 and Ti4SiC3 are the only 413 compound known to exist at present. A very large number of solid solution permutations and combinations are also conceivable as it is possible to form solid solutions on the M-sites, the A-sites and the X-sites of these different phases.
Michel Barsoum has synthesized, characterized and published data on the Mn+1AXn phases named above in bulk form [“The Mn+1AXn Phases: A New Class of Solids”, Progressive Solid State Chemistry, Vol. 28 pp201-281, 2000]. His measurements on Ti3AlC2 show that it has a significantly higher thermal conductivity and a much lower electrical resistivity than titanium and, like other Mn+1AXn phases, it has the ability to contain and confine damage to a small area thus preventing/limiting crack propagation through the material. Its layered structure and the fact that bonding between the layers is weaker than along the layers (as in graphite) give rise to a very low friction coefficient, even after six months exposure to the atmosphere. Polycrystalline samples however do not have such a low friction coefficient and tend to be brittle at room temperature.
Barsoum's synthesis process, for the Mn+1AXn phases he studied, involved the simultaneous application of high temperature and high pressure to starting materials in bulk form in a hot isostatic press. The starting materials react under pressure to produce Mn+1AXn phases. The methods used for producing epitaxial binary carbide films (such as chemical vapour deposition, CVD and physical vapour deposition, PVD) are carried out at high temperatures (1000-1400° C.). Films grown or synthesized at lower temperatures tend to be amorphous or compact grained. As yet there is no method that is able to produce crystalline/epitaxial thin films or coatings of Mn+1AXn at a relatively low temperature.